CMP is a well known process in the semiconductor industry used to remove and planarize layers of material deposited on a semiconductor device to achieve a planar topography on the surface of the semiconductor device. To remove and planarize the layers of deposited material, typical CMP, as depicted in prior art FIG. 1, involves wetting a pad 100 with a chemical slurry 110 containing abrasive components and mechanically "rubbing" or "buffing" a semiconductor device, usually found on a wafer 120, against the wet pad 100. The rubbing removes the layers of deposited materials on the front surface of the wafer 120 and planarizes the surface.
The planarizing is performed by rotating the carrier 130, holding the wafer 120, in a carrier rotational direction 140 and mechanically "rubbing" the wafer 120 against the pad 100. The wafer 120 rotates with the carrier 130 due to frictional forces between the carrier 130 and the wafer 120, in addition to a vacuum (not shown) within the carrier 130 holding the wafer 120. A platen 140 holds the pad 100 and rotates the pad 100 in a platen rotational direction 150 as the rotating wafer 120 is lowered in the contact direction 160 to contact the pad 100 and planarize the surface of the wafer 120. The types of deposited materials on the wafer 120 that are removed and planarized may include metal layers as well as dielectric layers that are on the front surface of the semiconductor device located on the wafer 120.
Certain relevant steps involved in the CMP of a metal layer, such as tungsten, are shown in prior art FIG. 2. FIG. 2 depicts cross-sectional views of the same semiconductor device portion undergoing certain relevant steps in the metal CMP of a tungsten metal layer 10. The tungsten metal layer 10 must be planarized down to a planarized level 40 above an insulator 45 by removing the excess tungsten metal 50. At the first step 60, the surface of the tungsten metal layer 10 has already been chemically converted by contact with a chemical slurry (not shown) to form a passivating layer 20. An oxidizing agent in the chemical slurry, such as ferric nitrate in the case of tungsten, chemically reacts with the tungsten to form the passivating layer 20 (tungsten oxide). The passivating layer 20 protects the recessed area 30 of the tungsten metal layer 10 from chemical erosion known as "dishing" since the recessed area 30 remains under the chemical slurry and would be subject to this dishing if not for the passivating layer 20.
In step 70, the passivating layer 20 has been removed by the mechanical rubbing, as discussed above in the CMP process, of the elevated area 35 of step 60. Step 80 depicts the reformation of the passivating layer 20 over the elevated area 35 as the CMP process continues since the chemical slurry is consistently being deposited on the surface of the tungsten metal layer 10 and thereby chemically reacting with the metal layer 10 to form the passivating layer 20. Steps 60 to 80 are repeatedly performed until the excess tungsten metal 50 is removed as depicted in step 90 where the CMP ends.
During this tungsten CMP, the recessed area 30 is well protected from any chemical recess by the chemical slurry into the tungsten metal layer 10, i.e. "dishing", by the tungsten oxide (passivating layer 20). Such dishing may lead to electrical problems (e.g. high line resistance) and undesirable topography of the device causing degraded device performance. Furthermore, the passivating layer prevents metal corrosion of the metal layer 10 which, if not prevented, can also lead to device failure. The passivating layer 20 therefore helps achieve a reliable device with excellent planarity, all at a high polishing rate.
While the formation of a passivating layer 20 for tungsten metal layers results in a reliable device, the formation of a passivating layer for a copper metal layer has particular problems--first, the copper oxide formed during copper CMP is not sufficiently passivating to prevent penetration of the chemical slurry into the copper metal layer. This is likely due to the fact that copper oxide may exist in two forms, Cu(I) oxide and Cu(II) oxide, which, when both forms are present in a given copper oxide layer, the copper oxide layer is porous in nature. A chemical slurry used during copper CMP forms a copper oxide, much like a tungsten oxide is formed for a tungsten metal, but unlike the tungsten oxide, the copper oxide does not sufficiently passivate the copper metal in the recessed area. This is because the chemical etching will occur in the recessed areas due to the penetration of the chemical slurry into the porous copper layer. As such, the copper oxide causes chemical recess into the copper layer thereby causing corrosion of the copper layer. This may result in device failure.
A further problem is that the removal rate of the copper oxide is not sufficiently fast enough to satisfy manufacturing requirements. Generally, a rapid removal rate in Angstroms per minute is very important to the design of a CMP process since a rapid removal rate increases the throughput of the wafers undergoing the CMP process. While the removal rate of tungsten oxide has achieved a satisfactory removal rate, such a removal rate for copper oxide has not.
Prior art literature has suggested that the addition to the chemical slurry of an inhibitor during copper CMP, such as benzotriazole (BTA), may assist in passivating the copper layer during the copper CMP. However, it is not clear whether the inhibitor provides uniform coverage over the entire copper layer, that is over both the elevated area and recessed areas of the copper surface, or whether the inhibitor allows for a rapid removal rate. BTA is known to form a complex with a copper ion having a positive one charge (Cu(I)) more readily than with a copper ion having a positive two charge (Cu(II)) at copper surfaces. Furthermore, amines are known to form complexes with Cu(II) and it has also been shown that the addition of both benzylamine and BTA to a chemical solution can enhance the anticorrosive ability of the solution on the copper surface, however the reason for this effect remains unclear. The effect of a Cu(I) and Cu(II) in the chemical slurry is not known.
A need therefore exists for a method of CMP of a copper metal layer that provides rapid removal rate of the copper oxide and provides a passivating layer over the entire copper surface to protect recessed areas from further chemical etching and/or copper corrosion during the CMP process. The chemical recess must be avoided since such a recess may lead to higher line resistance and undesirable topography which makes process integration difficult, causing degraded device performance and/or yield loss. Further, the copper oxide produced in the corroded copper can also lead to higher resistance and decreased yield. Finally, integratability of the inlaid back-end process depends on the ability to produce highly planar surfaces.